Can't remember the name of the movie

[Chris]

I saw it ages ago. It was about two brothers in a movie theater. They discover that it's been taken over by aliens.

I remember the ending where the main alien and his top henchmen are devoured by a giant trapdoor spider.

[Andrew]

I believe that you are looking for "Invasion Earth:  The Aliens Are Here" (1988).  The minor aliens are bipeds with insect-like features.  The main alien is a one-eyed creature with a trunk.  At the end the creature they create from the movie theater crowd's reaction eats them, and it looks a bit like a giant spider (many eyes, many legs, a big toothy mouth).